1. Field of the Invention
The present invention is directed to a magnetic head assembly of the type used in a high-speed tape drive to transfer data to and from a magnetic tape moving past the magnetic head assembly in the drive.
2. Description of the Prior Art and Related Applications
In high-speed tape drives wherein data is transferred to and from a magnetic tape moving at high speed in the drive, using a magnetic head assembly, the data are written in a large number of closely adjacent, or overlapping, data tracks. The tracks extend in a longitudinal direction parallel to the direction of tape transport, and are disposed side-by-side in a direction perpendicular to the transport direction. In such systems, it is known to employ the read head of the magnetic head assembly to verify the data which has been written in a pass by the write head, so that any errors in tracking or data transfer can be immediately identified, and corrective steps can be taken. A system wherein the verification takes place in the same pass as the writing of the data is known as a read-while-write system.
It has been observed and experimentally confirmed by the present inventor that if a track is recorded on a magnetic tape in a belt-driven cartridge using a write head, and if the tape in the cartridge is then rewound to exactly the same position as where the recording began and is then driven, the tape will exhibit nearly the same transverse tape movements (TTM) as the tape exhibited when the first track was recorded. Such transverse tape movements occur in a direction substantially perpendicular to the direction of tape transport. By means of a servo track or tape edge monitoring, the write head can thus be made to precisely follow these transverse movements so that the "guard bands" between tracks can be made small to prevent overwriting from track-to-track (or, if there is intentional track-to-track overlap, this overlap can be precisely controlled). The details of this phenomenon, and supporting experimental data, are set forth in U.S. Pat. No. 5,379,165, the teachings of which are incorporated herein by reference. The aforementioned phenomenon can be summarized as a recognition of the fact that there are strong cross-correlations among TTM waveforms obtained from repeated measurements at the same longitudinal position on the tape. Cross-correlation between two functions f.sub.1 (t) and f.sub.2 (t) is a well-known mathematical and engineering tool and is defined, for example, in the book by Papoulis, Anthanasios: "The Fourier Integral and its Application," McGraw-Hill Book Company, 1962, page 252, as the integral: ##EQU1## A graphical representation of such a TTM waveform as described in the aforementioned U.S. Pat. No. 5,379,166 is shown in FIG. 1. The upper waveform in FIG. 1 was the first one measured. It is designated f.sub.1 (t). The lower one was the 1000th measured, it is designated f.sub.1000 (t). The notation for the cross-correlation corresponding to the measurement reproduced in FIG. 1 is therefore .rho..sub.1,1000 (t), and it is very large. A slow shift or drift in the transverse direction over a relatively long time, as seen in FIG. 1, will not much disturb the basic temporal shape of .rho..sub.1,1000 (t). Even when sudden, unpredictable but not too large changes take place, for example, due to external mechanical vibrations or duration-limited and amplitude-limited shocks to which the tape drive is subjected, the temporal TTM waveform of the cartridge is maintained and the resulting TTM is a superposition of this waveform and the external one.
In conventional read-while-write head assemblies, a relatively long spacing exists between the write and read gaps, primarily for the purpose of avoiding crosstalk or other interference between the gaps. This relatively long distance results in a phase shift or phase difference between the TTM waveforms respectively measured at the position of the write gap and at the position of the read gap. In such conventional head assemblies, therefore, the inherent tracking repeatability of the write gap, upon playback of a previously recorded track, cannot be utilized to its fullest extent, since the read gap is not located at the same position as the write gap was located at the time the data was written, due to the aforementioned phase shift.
Moreover, the cross-correlation among TTM waveforms obtained from repeated measurements at the same longitudinal position of the tape ceases to exist, or is at least not reliably predictable, for very short TTM wavelengths. This is caused by random TTM stemming from physical phenomena other than "weaving" caused by the rotating parts of the cartridge, which gives rise to the TTM. Such random TTM may result, for example, from transverse-mode "string" vibrations of the tape, in the same way as the well-known longitudinal-mode Instantaneous Speed Variations (ISV) of the tape are caused by random frictional forces and the dynamics of the running tape. The minimum longitudinal distance at which cross-correlation among TTM waveforms can be seen will be referred to herein as the correlation distance, designated D.sub.corr.